Method of coating metal with aluminum



Patented M ar. 18, 1941 UNITED STATES PATENT OFFICE METHOD F COATINGMETAL WITH ALUMINUM Application August 13, 1936, Serial No. 95,940renewed July 24, 1940 7 Claims.

The present invention relates to a method of manufacturing a compositemetal structure consisting of a metal plate or strip having a coating ofaluminum.

It is an object of this invention to provide aluminum coated metal, suchas steel sheet or strip, which may be employed in lieu of tin plate nowgenerally used, in the manufacture of various articles, such ascontainer closures and metal containers, such as cans. In themanufacture of these articles there is now generally used a base steelsheet ranging from approximately .005 to .0115 inch thick; these gaugesare typical of steel sheet generally used in the container and closureindustry.

The present invention seeks to provide sheet steel of this characterhaving an aluminum coating which is superior to the tin coated steel nowused and which lacks the defects which have been generally recognized aspresent in aluminum coated steel as heretofore produced.

It is an object of the invention to provide aluminum coated sheet orstrip material which may be annealed, cold rolled to the desired gauge,and Vsubsequently drawn, pressed or stamped for making various articlesof the character described without 4irnpairing the characteristics ofthe aluminum coating.

It is an important object of the invention, that the coated metalarticle has a smooth continuous surface of aluminum highly resistant tocorrosion.

A further object of the invention is to provide an aluminum coatedsheet'or strip in which the aluminum coating has a permanent mirror-likeand lustrous appearance.

Another object of the invention is to form a coating which by reason of`the control of the iron-aluminum alloy formation will not break downunder coldvrolling or the high baking temperatures required when usingcertain types of decorating lacquers and venamels.

Another object is to provide a product relatively free from brittlenesswhich has heretofore characterized aluminumcoated steel due to thereaction between the iron and the aluminum, and Which maybe cold rolled;this iron-aluminum reaction forms an intermediate layer between thecoating and the base and sometimes at the surface itself and I havediscovered that it must be controlled in the coating process in orderthat the resulting product may be cold rolled and worked into closuresand similar articles. While this reaction cannot be entirely eliminated,it is controlled by this invention and the presence of the reactionproduct is so markedly reduced that it is not an objectionable factorWhere cold rolling, bending and shaping of the coated metal is required,as in the manufacture of containers and closures therefor.

I have discovered that in order to accomplish the foregoing objects itis necessary to maintain a careful control of the sheet or strip to becoated (a) immediately prior to its immersion in a coating bath, (b)during the immersion, and (c) immediately subsequent to removal of thecoated strip from the bath.

(a) Preliminary treatment.-Numerous attempts have been made to coatsteel with aluminum and other metals, and it has been generallyrecognized that after thoroughly cleaning the metal surfaces,mechanically or otherwise, it is desirable to subject the surfaces to areducing agent in the form of hydrogen or a hydrogen containing gas;examples of this procedure are to be found in such patents as No.825,219, Hyde, July 3, 1904; No. 979,931, Cowper-Coles, Dec. 27, 1910;No. 971,136, Monnot, Sept. 27,1910; British 4,313, of 1907, and British18,539, of 1907.

I have discovered that in order to obtain a product having the requiredcharacteristics, it is necessary to main-tain a critical control of thetemperature While subjecting the metal to the reducing influence of areducing gas containing hydrogen, and one feature of this inventionresides in the step of maintaining the temperature of the hydrogen orhydrogen containing gas Within a definite range hereinafter morespecically described. For coating metal of the character described, Ifind desirable a temperature not less than approximately 1000 F., and`this. temperature may run as high as 1600 F. or higher. The stripshould be subjected to these temperatures, preferaly while traveling,whereby to permit a continuous process, for a sufficient length of timeto raise the strip substantially to the temperature selected.- It hasalso been found that by utilizing this temperature it is possible toeffect simultaneously a desired heat treatment of annelaing of themetal. I find that temperatures in the neighborhood of 1225o F. aresatisfactory with a strip speed of 32 feet per minute and thistemperature should be increased as the strip speed is increased. It hasalso been found desirable to maintain an accurate control of the striptemperature between the hydrogen treatment and the immersion in thealuminum to avoid an objectionable drop in the temperature of the metalbefore immersion; preferably the strip should enter the bath at acoating of steel by immersion in a bath of hot aluminum forms betweenthe steel and the exposed aluminum surface an iron aluminum alloy(FeAla) and the nature of this alloy is discussed in the patent toOrtiz, No. 1,409,017, granted March 6, 1922. I have discovered that inorder to obtain a coating of a character which willenable the metal tobe cold rolled and formed into articles of the`character mentioned, itmay be continuously drawn through a bath of aluminum, the temperature ofwhich may range from approximately 1270 F. to 1325 F., provided thespeed of travel is regulated to maintain at a desirable minimumthickness the strata of iron aluminum alloy on both metal surfaces, andto prevent the formation ofl alloy areas or spots at the coatingsurfaces. that the period of immersion'be maintained at a minimumconsistent with the formation of a coating of the required thickness. Itis also preferred to maintain the surface of the bath, where the stripenters, free of oxide or slag nlm, and also to prevent the formation ofsuch a lm at the area of the bath where the strip makes its exit.

(c) Post-immersion treatment.-As the strip leaves the aluminum bath themolten aluminum on its surface has a bright mirror-like reflectivesurface, and unless the strip is carefully handled immediately after itswithdrawal from the bath, its surface becomes dull in appearance; I havediscovered that if the metal is rapidly cooled, it retains its brightreective surface to a marked degree, and that the coated surface thusobtained is much less susceptible to corrosive agents than a relativelydull slowly cooled surface. It is believed that in slow cooling. theiron which is present in all commercial aluminum lngot in the form ofFeAls, tends to segregate itself from the aluminum whereas by rapidcooling this iron is maintained well dispersed throughout. I find itdesirable to regulate the cooling in such a way as to maintain thisdesired dispersion and appreciably preserve the reflective surface ofthe molten aluminum which is apparent on the metal as it leaves thebath. I also prefer to direct the'strip as it leaves the bath along a4substantially vertical path for a considerable distance whereby tofacilitate the flow of excess molten aluminum from the strip back to thebath and tomaintain this flow uniform on both surfaces. Thispostimmersion treatment of the strip may be a'ccomplished by maintainingover the bath a positively cooled chamber through which the striptravels and having therein an inert gaseous medium such as nitrogen.Also, if desired there may be provided immediately over the bath areducing atomsphere of nitrogen or hydrogen, for example, thetemperature of which may be maintained approximately that of the bath,either by positive heating means or by radiation from the bath. Thestrip or sheet upon chilling of the coating may be continuously woundinto coils.

The aluminum coated metal may be flu'ther annealed; but it is desirableto maintain the annealing temperature below the melting point of thealuminum, and this will tend to prevent growth of the iron aluminumalloy.

The product may also be cold rolled to -the required gauge withoutobjectionably affecting the To accomplish these results it is desirablealuminum coating, and may be drawn, stamped,

Y or pressed into closures, containers, and similar articles.

'Ihe method thus broadly described may be carried out in various ways,and in order that the same may be more clearly understood, I haveillustrated in the accompanying drawing a. suitable form of apparatusfor practicing the method.

Referring to the drawing,

Figure 1 is a diagrammatic view of the apparatus used for carrying outthe method of this invention; and

Figure 2 is a fragmentary diagrammatic View of a modication,

In carrying out the invention, by way of illustration, I will describethe method in connection with steel strip or tin plate of varyingthicknesses, for example, .005", .010" and .015" and in some cases,strip having a thickness of .0115 is used. This steel strip will usuallyhave a carbon content of from .5 to .10%. 'I'he strip is fed from a rollI through a reducing atmosphere of hydrogen in the tube II and this tubeis exteriorly heated, as by a suitable electrical coil I2. Thetemperature of the tube I I is maintained above 1000 F., since I findthat at this temperature a smooth continuous coating is possible.Temperaturesas high as 1600 F. have been satisfactorily employed.v

In this preliminary treatment of the metal strip or sheet thetemperature factor is critical. I have operated successfully attemperatures from 1000 to 1600 F., but where the temperature is reducedappreciably below.1000 F. the coating has areas which are not properlyadhered to the steel and the strip has portions which are not otherwisecoated satisfactorily. 'Ihere does not appear to be any particular timeperiod necessary so long as the strip is raised to the predeterminedtemperature selected; it has been found that the strip may be travelledat relatively slow speeds through the tube II; for example, two feet aminute, or even at higher speeds, such as eight or sixteen feet a.minute, the speed being largely ldetermined by the period of travelthrough the aluminum bath through which the strip is continuously fedfollowing thisV preliminary treatment. I prefer temperatures in the tubeII of from 1300* F. to 1400" -F. since these temperatures permit adesirable heat treatment or annealing of the strip, which enhances thevcoating.

The entrance end of the tube II is closed by a suitable insulated discI3 having an opening through which the strip passes and the exit end ofthe tube II communicates with a chamber I4 Yin which is supported a rollI3' over whichthe strip travels to the bath. The walls of the chamber Ilterminate in a downwardly extending conduit I5 also filled withhydrogenfthe lower end of. the conduit extends into a pot y I6containing the molten aluminum which constitutes the bath. It will benoted that the strip is maintained out of contact with the airsubsequent to the heat treatment and before immersion and enters thealuminum bath in an atmosphere of hydrogen so that at this point nooxide is present on the surface of the bath.

The pot I6 is heated by a suitable furnace I'I so that the aluminum ismaintained fluid. Within thebath is disposed a roller I8 which isremovable and about which the strip -travels downwardly and upwardlythrough the bath. With respect to the bath, I prefer to arrange the sameso that the length and period of travel of the strip or sheettherethrough are at aminimum since I have found 'that the period andlength of `travel have a direct bearing upon the extent of iron-aluminumalloy structure of the product. The bath is preferably maintained attemperatures ranging from approximately 1275o F. -to l325 F., and it hasbeen found that the best results are obtained when the strip is moved ata speed of approximately eight feet a minute. a1- tliougli successfulresults have been obtained at speeds running from two feet a minute tosixteen feet a `1nnute. Generally a speed is selected which will producea thin coating with a tendency to form alloy strata on the strip ofminimum thickness, it being found that thick alloy strata tend toincrease the brittleness of the strip.

It is important, also, to control the temperature of the strip enteringthe bath, and it has been found that best results are obtained if thetemperature is not appreciably below 400 F. in the tube above the bath,i. e. at the time of entry; preferab1y`""t`he temperature of the stripbetween the heat treatment and the chamber I4 above the bath should notdrop below from 750 to 900 F.

'I'his temperature may be regulated'by varying the speed of` the strip,and an increase in speed tends to increase the temperature of the stripwhen it enters the bath. The thickness of the strip is also a factor indetermining the temperature, it having been noted that a strip ofapproximately .0115 has a temperature immediately above the bath, i. e.at the point of entry, of 400 F. with a speed of travel of eight feetper minute, whereas at the same speed a strip having a gauge of .015 hasa temperature of approximately 500 F'.

In order to facilitate determination of the strip temperature, means 26are located in the tube II for controlling the temperature and athermocouple 26 may be located in the chamber I3.

, The strip emerges through the bath top surface substantiallyvertically and is carried over a suitable roller I9. I prefer .to havethe strip emerge vertically from the bath and travel vertically for asubstantial distance, e. g., eighteen inches, since there is lesslikelihood of lumping, a more uniform coating is produced, and thepost-immersion treatment of the coated strip is facilitated.

The means for drawing the strip through the tube II, chamber Il and bathI6 may be one of l several devices, but I prefer a pair of pinch rolls20 which may be operated to vary the speed of movement of the stripthrough the tube and bath. After it emerges from the bath, the coatedstrip is Wound by means of a suitable coiler 2l. Between the roll I9 andthe pinch rolls 20 may be arranged a blower 22 for applying a blast ofcool air to the coated strip.

The strip as it emerges vertically from the bath has a brilliant moltensurface of aluminum which is hardened due to the cooling effect of theatmosphere as well as the roller I9 and blower 22. The roller I9 may bepositively cooled, as by internal water circulation, or any suitablecooling means may be disposed in close proximity to the bath so that thecoating is hardened by a positive cooling -as the strip with moltensurfaces leaves the bath. A positive cooling means tends to produce ahighly reflective finish and a preferred cooling means is shown inFigure 2.

Referring to Figure 2, the strip is led from the bath over a secondroller I8' and vertically up through a tube 29, the lower end of whichprojects into the bath. The tube 29 is in communication with the tube IIor the line 24 for receiving a., supply of hydrogenv at elevatedtemperature therefrom. In this manner, the molten aluminum about thelower end of the tube 29 is also maintained free of oxides so that at nopoint does the strip in its travel through the bath accumulate any ofthe usual oxide deposit. O-n emerging from the bath vertically into thetube 29, the strip enters an inert or reducing atmosphere of hydrogen,nitrogen, or other suitable gas, in the chamber 29a at the lower end ofthe tube 29; this chamber is at a temperature sumciently elevated tomaintain the aluminum coating molten and free from oxidation. The gasmay be heated, for example, by radiation from the coil 32 Iand. thebath. The strip then travels through the opening 30 in a partition 3l inthe tube 29 into a chamber 29h which may have a considerably reducedtemperature, i, e., a temperature such as will cause instant hardeningor freezing of the brilliant mirror-like surface and form a permanentsmooth coating of desired lustre. A reducing atmosphere containingnitrogen or hydrogen may be maintained in chamber 29a. In this manner,the strip is given a quick chill, and the cooling temperature will becontrolled by the temperature of the cooling medium in a coil 32positioned in the tube 29 as shown. This quick chill applied to thestrip will assure that the same will have a mirror-like lustre and thepreliminary travel through a hydrogen atmosphere at increasedtemperature will avoid formation of a dull or oxide surface. The stripis then led over the roller I9, which may be cooled as previouslydescribed, and then if desired, may be subjected to the blower 22 andthereafter wound as at 2|.

In some cases, the partition 3I is not employed, and the strip travelsvertically from the bath through an atmosphere of hydrogen maintained atelevated temperature through the medium of a heating coil 33 and fromthis zone, the strip then travels into the cooling Zone dened by thecoil 32 where the cooling and consequent hardening of the molten coatingis carried out in a more gradual manner. It is likewise desirable thatthe strip be travelled through the tube 29 at a rapid rate in order toprevent reaction of the iron and aluminum growth of the alloy strata. Inthis connection, the temperature of the hydrogen is controlled so as tomaintain the coating molten without promoting such reaction or growth ofthe reaction product. 'I'he cooling of the molten strip inhibits theiron-aluminum reaction.

In the case of a continuous strip, the speed of the pinch rolls 20 willbe adjusted so as to travel the strip through the severalinstrumentalities at the required speed.

I prefer to travel the strip through the aluminum bath at a relativelyhigh speed such as eight feet per minute, since I find that at thehigher speeds at which the strip is introduced into the bath,l thinnercoatings are produced and there is less opportunity for reaction betwenthe iron and aluminum. 1

Referring again to the preliminary treatment,

the tube I I is made of a length and the heating coil surrounds the samefor a sufcient distance, such that required temperatures for treatingthe metal with hydrogen and annealing the'metal are possible whilepassing the sheet through the tube II at the desired speed. It will beobserved that the strip undergoes a cooling as it passes from theconfines of the coil I2 into the chamber I4 and to the bath and thespeed of the strip is controlled so that the temperature drop is -notobjectionable in order that the advantages above described may beobtained. Ii' desired. the heating means I2 may be extended throughoutthe length of the tube II and about the-chamber I4 and tube I5. Byhaving the tube I5 disposed below the level of the bath in the pot I6,no oxide iilm will form at thesurface of the bath where the hydrogen isheated to about 1300 F. and the strip is carried through the tube at aspeed of about eight feet per minute. The strip at the point of entryinto the bath registers by means of the thermo-couple 26 atemperatureoi' about 400 F. In the case of steel strip having a thickness of .015",the strip enters the bath at a temperature of about 500 F.

At this speed, the strip is carried through the bath having atemperature of about 1325" F. and a coating was produced which was lessthan 0.0012.

The resultant coated sheet treated either ln accordance with Figure 1 orFigure 2 may be cold rolled in several passes to reduce its thicknessdown to, for example, .003" with no breakage of the surface or the bond,the coating remaining perfectly intact.

Where a lustrous or mirror-like surface is desired, the sheet iswithdrawn from the bath as shown and described in connection with Figure2, rst through a reducing and non-oxidizing atmosphere, such ashydrogen, at elevated temperatures, and then through a quick coolingzone to harden the molten' coating and preserve the lustre. Instead ofusing hydrogen in the tube 29, any'non-oxidizing gas such as nitrogenmay be employed. Y It is to be noted in connection with Figures 1 and 2that the strip is maintained out of contact with the atmosphere eitheruntil it emerges from the bath or until it emerges from the tube 29.

Thus the heating and cleansing with hydrogen?, the annealing step, andthe coating and subsequent hardening steps are conducted as a continuousoperation. If desired, the process may be carried out with sheets ofdenite size,'and it is to be understood that the thicknessV of thesheets and the strip material may vary as well as the aluminum coatingapplied, in accordance with the product desired.

Before operating the apparatus it is desirable that the tube I I and potI6 be flushed with nitrogen from tank N, the same being admitted throughline I2 and entering the tube by a suitable valve 25. After the`apparatus has been flushed with nitrogen, the hydrogen is then run intothe tube from the container H, but is rst preferably carried through acatalytic chamber 21 for removing oxygen and through the chambers 28where any moisture is taken out, the hydrogen then entering the tube IIthrough valve 25 hs heretofore explained, the product resulting fromthis method may be further annealed if desired. and may also be coldrolled. t has been established that this product is capable of takinghigh rolling reductions without any detrimental efl'ects. I have coldrolled material originally .013" thick in several passes down to .003"without breaking or objectionably affecting the coating. When thethickness of the FeAl: alloy is controlled in the manner described, theproduct may be cold rolled and suitably worked as by drawing orstamping, for the manufacture of apparatus illustrated and the detailsof the method disclosed are merely illustrative and that such variationsin the method and apparatus are within the invention as come within thescope of the appended claims. j

Whatisclaimedasnewis: l. In the art of manufacturing aluminum coatedsheet steel the method which comprises continuously traveling andheating sheet steel in a reducing atmosphere for a suilicient length oftime to raise the temperature of the metal to at least approximately1300 F., reducing the metal to a point between substantially 400 F. and900 F. in a non-oxidizing atmosphere and introducing the metal withoutexposure to oxidizing iniiuences into a bath of aluminum.

2. In the art of manufacturing aluminum coated sheet steel the methodwhich comprises continuously traveling and heating sheet steel in vareducing atmosphere for a suiiicient length of time to raise thetemperature of the metal to at least 1300 F., continuing the travel ofthe metal and simultaneously reducing the temperature thereof to a pointbetween substantially 400 F. and 900 F. and maintaining the Ametal whilecooling in its travel from the heating means in a non-oxidizingatmosphere, and introducing the metal continuously from the coolingmeans to an aluminum-bath, while maintaining .a non-oxidizing atmosphereabout the metal and said bath.

3. In the art of manufacturing aluminum coated sheet steel the methodwhich comprises continuously traveling and heating sheet steel in areducing atmosphere fora sufficient length of time to raise thetemperature of the metal to at least approximately 1300 reducing themetal to a point between lsubstantially 400 F. and 900 F. in anon-oxidizing atmosphere and introducing the metal without exposure tooxidizing iniiuences into a bath of aluminum. removing the aluminumcoated steel from said bath and cooling the'same by playing a blast ofair upon the sheet.

4. In the art of manufacturing aluminum coated sheet steel the methodwhich comprises continuously traveling and heating sheet steel in areducing atmosphere for a sulcient length of time to raise thetemperature of the metal to at least approximately 1300 F., reducing themetal to a point between substantially 400 F. and 900 F. in anon-oxidizing atmosphere and introducing the metal without exposure tooxidizing influences into a bath oi' aluminum, re-

moving the aluminum coated steel from said bath, and heating the same ina non-oxidizing atmosphere to maintain the coating molten, and thengiving the coating a quick cooling in a nonoxidizing atmosphere.

5. In the art of manufacturing aluminum coated sheet steel the methodwhich comprises continuously traveling and heating sheet steel in areducing atmosphere for a sufficient length of time to raise thetemperature of the metal to at least approximately 1300 F., reducing themetal to a point between substantially 400 F. and 900 F. in anon-oxidizing atmosphere and introducing the metal without exposure tooxidizing influences into a bath of aluminum, removing the aluminumcoating steel from said bath and cold-rolling the same.

6. In the art of manufacturing aluminum coated sheet steel the methodwhich comprises continuously traveling and heating sheet steel in areducing atmosphere for a sufcient length of time to raise thetemperature of the metal to at least approximately 1300" F., reducingthe metal to a point between substantially 400 F. and 900 F. in anon-oxidizing atmosphere and introducing the metal Without exposure tooxidizing inuences into a bath of aluminum, the temperature of the steelintroduced in the bath and the period of immersion in the bath beingjsuch as to retard formation of FeAls whereby the aluminum coated steelmay be cold-rolled and removing the aluminum coated steel from the bathand cold-rolling the same.

7. In the art of manufacturing aluminum coated sheet steel, the steps ofcontinuously passing a strip of steel through a heating means andheating the strip therein to a temperature of at least approximately1300" F. in a non-oXidizing atmosphere, then while maintaining the sheetin a non-oxidizing atmosphere, cooling the same while traveling to analuminum bath to a temperature of between substantially 400 F. and 900F., continuously introducing the strip from the cooling means to thealuminum bath while maintaining the strip and the bath in a nonoxidizingatmosphere, continuously passing the strip through the bath, and afterthe strip emerges, hardening the coating and cold-rolling the aluminumcoated sheet.

MATTHEW SCHON.

